A nymphomaniac. A woman who loves sex so much that her cunt is almost always red and swollen. The snapper part of the expression referring to how tight and eagerly her cunt seems to snap shut on cock. Also in part a fisherman's pun in that red snapper fish do not let go of the bait easily even after being hauled aboard the fishing boat (in this case the worm is a cock). Red snapper is also a tasty fish dish.... Thus again referencing very agressive nymphomaniac behavior.

I found me a red snapper at the bar last Friday night and I am so sore now. I barely got rid of her before going to work Monday.

A famous early 1980s Ron Jeremy porn flick is about the "legend of the red snapper".